real engines · reversible to the byte

Operate the machine, not the metaphor.

Step, break, and rewind real distributed-systems engines — down to the bytes on a simulated disk.

Start the learning path →Browse 24 systems

24 systems−1 → 10 levels0 hand-waving

Browse the systems

⌘K to search · or follow the −1 → 10 path

01

Storage engines

How databases persist and find data on disk.

4 systems

Skip ListSorted set, express lanesYou’llInsert keys and watch coin-flips build the express lanes for fast search.intermediatelvl 1 · /skiplist B+ TreeOrdered index behind SQLYou’llInsert keys and watch the tree split to stay balanced and sorted.intermediatelvl 1 · /btree BitcaskOn-disk key–value storeYou’llAdd each write to the end of a file, then find any value from a map in memory.intermediatelvl 2 · /bitcask LSM TreeWrite-optimized storage engineYou’llWrite keys and watch them flush to disk, then merge as the engine compacts.advancedlvl 2 · /lsm

02

Caching

Make reads fast — and keep the cache honest.

2 systems

LRU / LFU CacheIn-memory eviction policiesYou’llFill the cache past its limit and watch what gets dropped under LRU vs LFU.beginnerlvl 3 · /cache Caching StrategiesCache read/write patternsYou’llRun cache-aside vs write-through and watch a stampede form, then break it.intermediatelvl 3 · /cachestrategy

03

Indexes & routing

Find data and steer traffic on a single node.

7 systems

Trie AutocompletePrefix tree for typeaheadYou’llType a prefix and watch completions light up down the tree.beginnerlvl 1 · /trie Event LoopSingle-threaded server modelYou’llQueue tasks and step the loop to see how one thread stays busy.intermediatelvl 4 · /eventloop Rate LimitingAPI request throttlingYou’llSend bursts of requests and watch the bucket allow or deny each one.beginnerlvl 4 · /ratelimit Circuit BreakerFailure isolation patternYou’llTrip the breaker with failures, then send canaries to test recovery.intermediatelvl 4 · /circuitbreaker QuadtreeSpatial “near me” indexYou’llDrop points on a map and watch the grid subdivide where they cluster.intermediatelvl 6 · /quadtree Load BalancerTraffic spreaderYou’llSend requests and watch strategies route around a slow backend.beginnerlvl 6 · /loadbalancer Message QueuePartitioned durable logYou’llProduce messages and watch consumers track offsets and rebalance.intermediatelvl 9 · /queue

04

Distributed systems

Coordinate many machines when clocks and networks lie.

9 systems

Consistent HashingHash ring for serversYou’llAdd and remove nodes and watch keys move to only their neighbors.intermediatelvl 6 · /chash Kademlia DHTCoordinator-free DHTYou’llLook up a key by XOR distance and watch it converge in O(log n) hops.advancedlvl 6 · /kademlia Clock SynchronizationDistributed clock alignmentYou’llDrift two clocks apart, then run sync and watch them pull back together.intermediatelvl 7 · /clocksync Gossip ProtocolCoordinator-free replicationYou’llSpread an update node to node and watch the cluster converge.intermediatelvl 7 · /gossip Merkle TreeHash tree for diffingYou’llChange one replica and find the diff in a handful of hash comparisons.intermediatelvl 7 · /merkle Quorum ReplicationTunable replica consistencyYou’llSet R, W and N and watch reads go stale or get repaired.advancedlvl 7 · /quorum Raft ConsensusLeader-based consensusYou’llElect a leader, replicate a log, then crash it and watch a new one win.advancedlvl 7 · /raft Two-Phase CommitAtomic cross-service commitYou’llRun a commit across services, then crash the coordinator and watch everyone block.intermediatelvl 8 · /twopc Saga PatternCompensating transactionsYou’llRun a multi-step transaction, fail a step, and watch it roll back in reverse.intermediatelvl 8 · /saga

05

Probabilistic structures

Trade a little accuracy for a lot of memory.

2 systems

Bloom FilterProbabilistic membership setYou’llAdd items, then ask “is this in the set?” — and see why it can be wrong.beginnerlvl 2 · /bloom HyperLogLogApproximate distinct countYou’llAdd millions of items and estimate the count from a few kilobytes.intermediatelvl 10 · /hll